This invention relates generally to the operation of cryogenic tankage and is particularly useful for reducing flash off losses from cryogenic liquid stored in such tankage.
Cryogenic liquids, such as liquid argon, are transported from production facilities to the point of consumption. Losses of the cryogen are incurred as a result of heat leak into the cryogenic liquid during transportation as well as transfer of liquid into, and storage of liquid within, a storage facility near the point of consumption. The heat leak causes evaporation of some of the cryogenic liquid resulting in a pressure increase within the container to the point at which the vapor is vented to the atmosphere through safety valves. The heat leak into the cryogenic liquid not only causes some of the cryogenic liquid to vaporize, but also results in the liquid becoming warmer thus increasing flash off losses when the cryogenic liquid is passed from the storage facility to the use point.
Those skilled in the art have addressed this problem by using a relatively less expensive cryogenic liquid to condense evaporated cryogenic liquid. For example, by boiling liquid nitrogen against gaseous argon that evaporated because of heat leak, the argon is condensed and thereby recovered. The evaporated nitrogen is then vented to the atmosphere. In effect this is an exchange of relatively less expensive cryogenic liquid for a relatively more expensive cryogenic liquid. However, since liquid nitrogen, its storage and its use still entail considerable costs, the cryogenic liquid exchange method described above has shortcomings.
Accordingly, it is an object of this invention to provide an improved system for refrigerating the contents of tankage containing cryogenic liquid in order to reduce losses resulting from heat leak into the tankage.
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is:
A method for refrigerating the contents of tankage containing cryogenic liquid comprising:
(A) providing tankage containing vapor and cryogenic liquid, and passing vapor from the tankage to a heat exchanger;
(B) condensing at least some of the vapor within the heat exchanger by indirect heat exchange with refrigeration bearing refrigerant fluid to produce condensed vapor;
(C) subcooling the condensed vapor by indirect heat exchange with the refrigeration bearing refrigerant fluid to produce cryogenic liquid; and
(D) passing subcooled cryogenic liquid from the heat exchanger to the tankage.
Another aspect of the invention is:
Apparatus for refrigerating the contents of tankage containing cryogenic liquid comprising:
(A) tankage comprising at least one tank, a heat exchanger, and means for passing vapor from the tankage to the heat exchanger;
(B) a refrigeration system comprising means for producing a refrigeration bearing refrigerant fluid;
(C) means for passing refrigeration bearing refrigerant fluid from the refrigeration system to the heat exchanger; and
(D) means for passing fluid from the heat exchanger to the tankage.
As used herein, the term xe2x80x9cindirect heat exchangexe2x80x9d means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other.
As used herein, the term xe2x80x9cexpansionxe2x80x9d means to effect a reduction in pressure.
As used herein, the term xe2x80x9cexpansion devicexe2x80x9d means apparatus for effecting expansion of a fluid.
As used herein, the term xe2x80x9ccompressionxe2x80x9d means to effect an increase in pressure.
As used herein, the term xe2x80x9ccompressorxe2x80x9d means apparatus for effecting compression of a fluid.
As used herein, the term xe2x80x9cmulticomponent refrigerant fluidxe2x80x9d means a fluid comprising two or more species and capable of generating refrigeration.
As used herein, the term xe2x80x9cvariable load refrigerantxe2x80x9d means a mixture of two or more components in proportions such that the liquid phase of those components undergoes a continuous and increasing temperature change between the bubble point and the dew point of the mixture. The bubble point of the mixture is the temperature, at a given pressure, wherein the mixture is all in the liquid phase but addition of heat will initiate formation of a vapor phase in equilibrium with the liquid phase. The dew point of the mixture is the temperature, at a given pressure, wherein the mixture is all in the vapor phase but extraction of heat will initiate formation of a liquid phase in equilibrium with the vapor phase. Hence, the temperature region between the bubble point and the dew point of the mixture is the region wherein both liquid and vapor phases coexist in equilibrium. In the preferred practice of this invention the temperature differences between the bubble point and the dew point for a variable load refrigerant generally is at least 10xc2x0 C., preferably at least 20xc2x0 C. and most preferably at least 50xc2x0 C.
As used herein, the term xe2x80x9csubcoolingxe2x80x9d means cooling a liquid to be at a temperature lower than the saturation temperature of that liquid for the existing pressure.